Impact reveals lunar water by the bucketful

Lunar material kicked up by the LCROSS mission contained at least a few bucketfuls of water, NASA announced on Friday. The discovery bolsters the case for significant amounts of water at the moon's poles, a potential resource for human explorers.

Permanently shadowed craters on the moon are among the coldest known places in the solar system and have long been suspected to hide significant water deposits, a potential resource for future lunar outposts.

To hunt for signs of water, NASA's Lunar CRater Observation and Sensing Satellite (LCROSS) mission sent a spent rocket stage on a collision course with a 98-kilometre-wide crater called Cabeus on the moon's south pole on 9 October.

The impact excavated a 20-metre-wide crater in the floor of Cabeus, kicking up plumes of lunar material. Earth-based observers had trouble seeing the ejecta, because the densest part of the debris cloud flew low across the lunar surface and was hidden by a high ridge.

But LCROSS's shepherding spacecraft, which followed four minutes behind the rocket stage, caught a clear glimpse of material thrown up by the impact. It found spectral evidence for water in ultraviolet and infrared light measured by two separate instruments on the spacecraft.

'Definitive discovery'

"I'm here today to tell you that indeed, yes, we found water. We didn't find just a little bit. We found a significant amount," LCROSS principal investigator Anthony Colaprete of NASA's Ames Research Center in Moffett Field, California told reporters on Friday.

Though the total amount of water blasted out of the crater is uncertain, the LCROSS spacecraft detected more than 100 kilograms in the part of the plume it observed. It will take more modelling of the impact to estimate the concentration of the water in the crater.

Evidence that the moon's poles may contain stores of water ice has long been mounting. Radar signals bounced off a crater on the moon's south pole by the US Clementine spacecraft in 1994 hinted at the presence of water ice. And maps of slow-moving neutrons collected by NASA's Lunar Prospector, which orbited the moon in 1998 and 1999, suggested a number of craters are rich in hydrogen, though it was not clear whether the hydrogen was bound in water molecules or was present in some other form.

LCROSS's spectral measurements may clinch the case. "I think because of the ambiguity of the measurements there was no firm consensus in the scientific community that [the hydrogen] was in fact water. LCROSS has now made that definitive discovery," said Greg Delory of the University of California, Berkeley, who is not a member of the LCROSS team.

Comets or solar wind

Now researchers can now move on to tackle even bigger questions, Delory said. Among the biggest is where the water came from. One possible source is water-bearing comets that slammed into the moon.

Evidence for such an origin might be bolstered if scientists find signs of other materials known to be present in comets, like carbon-based compounds called organics. The LCROSS instruments registered some possible spectral hints of such materials, though that remains uncertain as the team continues to analyse the results.

Alternatively, the water at Cabeus may have been created when hydrogen atoms carried by the solar wind slammed into oxygen-rich materials in the lunar surface. Over time water molecules may migrate, hopping across the lunar surface before becoming trapped in cold craters at the moon's pole.

In September, scientists for India's Chandrayaan-1 and two other spacecraft announced the discovery of trace quantities of water over much of the moon's surface, which may be created in this process.

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